(Blender 2.9) Viewport Cycles render progress bar
Open the Viewport Overlays popover in the upper right of 3d viewport, to the left of the Viewport Shading Mode buttons, the downward arrow next to the two overlapping circles.
Check Statistics. If it doesn't appear, try going to a different display mode (Wireframe or Solid) then back to Rendered Mode.
You should then see render progress, as well as the number of objects, vertex, edges, faces and triangles in your seen in the upper left.